Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
In many automatic transmissions, the power flow paths associated with various speed ratios are established by engaging a subset of the available clutches and brakes. Typically, the clutches and brakes are engaged by providing fluid at a controlled pressure to a clutch apply chamber. The fluid pressure forces a piston which in turn compressed a clutch pack of alternating friction plates and separator plates. A controller typically regulates the fluid pressure by adjusting the magnitude of an electric current to a solenoid which regulates a force on a valve to adjust the pressure. To switch from one ratio to another ratio, one clutch or brake is released and another clutch or brake is engaged. Some shifts require release of multiple clutches and engagement of multiple clutches.
Making a smooth shift between ratios requires that release of the off-going clutch and engagement of the on-coming clutch be gradual and carefully coordinated. The relationship between the current supplied to the solenoid and the clutch torque capacity is subject to many noise factors. Therefore, accurate control of the clutches requires use of a feedback signal.